PIDs usually incorporate a chamber through which a sample gas is passed by means of suitable tubing, pumps, fans and the like. The chamber is exposed to photons. A proportion of the photons has sufficient energy to disassociate any photoionisable gas molecules within the sample into ions. This process is known as photoionisation and the chamber itself is described as a photoionisation chamber. Each photoionisation event engages one photon and causes equal numbers of positively and negatively charged ions to form, usually one of each. An electric field is applied across the chamber by means of two or more electrodes which are a part of, or are contained within, the chamber walls. The ions are attracted to the electrodes, causing a current to flow between the two or more electrodes within the chamber. This current is amplified and displayed, providing an indication of the presence of the target gas molecules.
PIDs also incorporate means to produce UV light of sufficient energy to ionise gases of interest, such as volatile organic carbon compounds, without ionising common constituents of clean air, such as nitrogen, oxygen, argon and/or carbon dioxide. UV discharge electrodeless lamps of up to a few centimeters length and up to about one centimeter diameter are commercially available. These lamps contain a gas at a few millibars of pressure, typically a noble gas such as krypton. Soda glass is a suitable material for the body of the lamps, terminating at one end with a wall which typically comprises a flat round disc of magnesium fluoride and lithium fluoride which are transparent to photons in the energy range of interest and which typically forms one wall of the ionisation chamber.
It is desirable to remove and thereby detect photogenerated ions in a photoionisation chamber by means of electrodes located no more than a millimeter or two from the lamp face and from each other. The reasons for this are fourfold. First, photoionising UV light may be adsorbed not only by photoionisable gases but also by other gaseous constituents of air, most significantly, moisture. Thus an electrode network extending more than one or two millimeters from the lamp face gives rise too readily to a response to ionisable gas within air which varies with humidity. Second, the absorption of light by ionisable gases increases with distance from the lamp, causing the response to an organic gas not to increase in proportion to its concentration, even at quite modest concentrations (tens of parts per million of ionisable gas) in chambers having an electrode gap of more than a few millimeters. Third, a few hundred volts per mm of electrode gap must be applied between the electrodes in order to strip out all the photo-ions between the electrodes before they are conveyed by sample flow away from the electrodes or recombine to form neutral molecules which are not attracted to the electrodes and do not contribute to the photoionisation current. Generating such a voltage becomes more problematic as the gap across the electrodes is increased. Finally, positive and negative ions are more likely to encounter each other and recombine as the electrode gap is increased, again causing poor proportionality between response and ionisable gas concentration.
The electrodes are typically of planar form and have thicknesses less than a few tenths of a millimeter. One electrode, namely the anode, is at a positive electric potential relative to the other electrode, namely the cathode, and abuts or is otherwise located very close to the lamp window, attracting negatively-charged ions. The anode includes means for admitting light through it, for example, by slots or holes. The cathode is at a negative potential relative to the anode and is placed parallel to, and within a millimeter of, the anode attracting positively-charged ions. This arrangement assures that positive ions, which form more contaminating products than do negative ions, are conveyed away from the lamp window.
However, some ionisable gases form negatively charged ions which are attracted towards the lamp windows and, are adsorbed thereon. Further, collisions between such negative ions and neutral molecules within the ionisation chamber can generate ion clusters which, being of high molecular weight, are very prone to deposition on surfaces, such as the lamp window. Further, the sample gas might contain non-volatile material in aerosol form which can also deposit on the lamp window. In short, the lamp window can be fouled by species within the sampled air, whereby the sensitivity of the PID to a given concentration of ionisable species is reduced. Most ionisable gases are organic and, therefore, it is reasonable to anticipate that window fouling is usually caused by a film of organic material. Sometimes, a photoionisation chamber may also be fouled by liquids being drawn into it.
It will be apparent that photoionisation chambers are frequently of small dimensions and, in certain applications, prone to contamination or mechanical damage during use and servicing. It is thus desirable to design the photoionisation chamber to be readily replaceable. Such replaceable chambers will include electrical terminals to the photoionisation electrodes described above. Other gas sensing ionisation chambers might also be advantageously rendered easily replaceable for similar reasons.
However, there has frequently been difficulty in assuring good electrical contact between the electrode terminals in the replaceable chamber and terminals from supporting electronic circuitry contained within a sensing member which accommodates the replaceable ionisation chamber. In particular, spring-loaded retractable pins, which ensure a force is applied between the contacts, do not always retain their spring loading. Also, pins inserted into metal or metal-coated holes are prone to failure after multiple engagement and removal of the ionisation chamber.
The invention seeks to mitigate the problems and disadvantages associated with the known electrical contacts of disposable PID electrode assemblies discussed above.
Needs dictate that the small detector pellets of such assemblies have five external connections located within close proximity of each other, to allow the working electrodes, as previously described, maximum space to collect the charged ions in the operating chamber part of the detector pellet. Five connections from an external printed circuit board (PCB) to the respective connections located on this pellet have to be constructed in such a manner that manufacturing tolerances in contact alignment in three dimensions are readily overcome.
One form of prior art arrangement uses spring-loaded telescopic pins which can change their length to take up manufacturing tolerances in one dimension and are designed so they depress further to provide firm contact on preformed metal contacts located within the pellet against spring pins. They rely upon an internal, suitably small, coil spring located within the assembly. This technique requires much space and if specialised miniature telescopic pins are used, then the costs may be prohibitive.
Preformed contact surfaces in the pellet may be used to bring all the differently placed electrode contacts on to one common plane for the spring-loaded telescopic pins to press upon.
There are several technical problems with this approach.
First, the expensive telescopic pins are, for practical reasons, located within the retained PCB exterior to the disposable pellet. The whole assembly can be used in aggressive and potentially corrosive atmospheres and thus, after some repeated use, the pins may become damaged or corroded, thereby losing their effectiveness in communicating electrical signals between the pellet and the externally retained PCB. The prior art arrangements make no allowances for this and thus disposal of the pellet together with the whole externally retained PCB must be effected.
A further problem with this prior art arrangement is at the points of contact between the pins and contacts.
These points of contact are generally between the pins and contacts which are perpendicular to each other and, thus, have no means for wiping the points of contact clean whilst making contact between the pins and contacts. Oxide or external debris if not wiped away during the contact-making generation may impair good ohmic contact for electrical signals.
Another prior art arrangement employs a pin and slide socket arrangement. The pin is located within the electrode pellet parallel to the direction of pellet placement with respect to the mounting PCB and slides into a suitably placed socket located on the PCB. This has the advantage of being slightly less expensive than the prior art arrangement discussed above and the barrel action of the interference spring connection within the socket allows the slide sockets to be packed more tightly together.
Advantages of this prior art arrangement over the previous one described above will now be explained.
First, there is a wiping action while contact is being made which aids oxide and debris removal and, hence, enhances ohmic contact.
Second, provided the socket is long enough in its sliding contact length, then a common pin may be connected to the differently placed grids, thus relying upon the comparatively long travel of contact engagement to take up both manufacturing tolerances and electrode grid position.
However, this particular prior art arrangement does have some disadvantages.
The main disadvantage is that the socket and pins must be parallel to each other and to the direction of their relative travel. Clearly, this is not too much of a problem if it is just one or two pins, as in the prior art example, but when, say, five pins are used then this becomes more difficult to achieve. Further, this option still tends to be unduly expensive in relation to unit costs and assembly requirements.